As a material for apparatus for oil wells for use in the boring of crude oil, natural gas and the like, carbon steel and a corrosion suppressing agent (an inhibitor) were generally used in combination in the past. In recent years, due to the change in boring circumstances in accordance with the development of increasing of depth in oil well boring, higher corrosion resistance and mechanical properties (e.g., hardness) than before have been gradually required in materials for apparatus for use in oil wells, and thus steel materials having excellent corrosion resistance (alloy steels) have been used. For example, since the corrosion resistance of iron (Fe) is significantly improved by adding chromium (Cr), many martensitic stainless steels (e.g., SUS 420) containing 13% by mass of Cr have been used in oil wells containing metal corrosive components.
However, SUS 420 has a disadvantage that it easily causes stress corrosion cracking (SCC) under a circumstance containing a chloride and an acidic gas (e.g., carbon dioxide gas and hydrogen sulfide). Therefore, in the case when oil well boring is conducted under such hard corrosion circumstance, expensive nickel (Ni) based alloys (e.g., an alloy containing 40% by mass or more of Ni) have been conventionally used in many cases, and thus there was a problem that the material cost (eventually boring cost) significantly increased.
On the other hand, Cr based alloys are exemplified as corrosion resistant, heat-resistant alloys that are more inexpensive than Ni based alloys, and various Cr based alloys have been proposed. For example, PTL 1 (JP H04-301048 A) discloses a Cr—Fe-based heat-resistant alloy having a chemical composition including Cr: 65 to 80%, Co: 10 to 15%, Fe and impurities as a balance, and including N: 0.1 to 1.5% as necessary. PTL 2 (JP H04-301049 A) discloses a heat-resistant alloy having a chemical composition including Cr: 70 to 95%, N: 0.1 to 1.5%, Fe and impurities as a balance. According to PTLs 1 and 2, the alloys are deemed to be excellent in compression deformation resistance, oxidation resistance and the like in a high temperature atmosphere furnace, and thus significantly contribute to improvement of durability as a supporting surface member for a steel material to be heated, and decreasing of maintenance and accompanying improvement of a furnace operation efficiency.
PTL 3 (JP H08-291355 A) discloses a Cr based heat-resistant alloy containing, by mass %, Cr: more than 95% and N: 0.1 to 2.0%, as a balance, one kind or two or more kinds of Fe, Ni and Co, and unavoidable impurities, and further containing, as necessary, one kind or two or more kinds of Ti, Al, Zr, Nb, B and V by 0.3% or more in total. According to PTL 3, it is deemed that a Cr based heat-resistant alloy having an excellent high temperature strength, which is used in members for which strength, ductility and corrosion resistance are required under a ultra-high temperature (e.g., a supporting member for a steel material to be heated in a heating furnace) can be provided.
Furthermore, PTL 4 (JP H07-258801 A) discloses an Fe—Cr—Ni alloy including Cr: 15 to 50%, Ni: 6.1 to 50%, 0+P+S: 200 ppm or less, and Fe and unavoidable impurities as a balance, having a crystal particle size number: 8 or more, and containing C+N: 400 to 1,200 ppm as necessary, which gives excellent corrosion resistance to processed parts. According to PTL 4, it is deemed that an Fe—Cr—Ni alloy that improves corrosion resistance without decreasing processability, and does not decrease corrosion resistance even being processed, can be provided.